This invention relates to the field of signal identification, and more particularly to the field of recognizing a user defined sequence of events in serial digital data on one or more lines.
Recognizing the presence of a particular combination of logic states across a number of digital signal lines is not new. Logic analyzers have utilized word recognizers that perform this function for many years, since the earliest logic analyzers appeared on the market in the 1970s.
As logic analyzers have become more sophisticated and complex over the years, the ways in which a desired trigger condition can be defined have multiplied, and now include elaborate triggering facilities. Many of the triggering facilities present in modern logic analyzers include some sort of sequential state machine that ascertains when the signals under test have reached a particular condition after a complex series of other conditions have occurred in a particular sequence.
Logic analyzer triggering, complex though it has become, is based on separate and discrete views into the system under test which only occur at the time of a sampling clock. While some logic analyzers can augment this limited view with glitch detection, glitch detection usually only consists of ascertaining whether multiple transitions of the signal in question across the reference threshold occurred between sample clocks and does not resolve timing details about the detected activity that qualified as a glitch.
Oscilloscope triggering was for many years generated by the excursion of a signal under test past a preset amplitude threshold in a particular direction. Recently, oscilloscope triggering has grown to include the word recognition concept from the field of logic analysis. In this approach, a digital trigger section which includes a word recognizer monitors a digital version of multiple input channels waiting for a particular combination to appear before a trigger is generated. Oscilloscopes that can be triggered in this fashion are very useful in a modern digital environment, because it is often necessary to have an analog look at the activity of a signal during a time that is most easily specified in terms of the digital condition of a number of different signals. Refer to U.S. Pat. No. 4,585,975, to Wimmer for a "High Speed Boolean Logic Trigger Oscilloscope Vertical Amplifier with Edge Sensitivity and Nested Trigger", which discloses the combination of a digital trigger path with an analog signal path.
Oscilloscope triggering was further advanced by the disclosure in U.S. Pat. No. 4,748,348, to Thong, for a "Multi-Level Pattern Detector for a Single Signal". Thong disclosed a method and apparatus for detecting the occurrence of a plurality of selected sequential events in a single monitored signal. However, Thong's approach has a limitation which it would be desirable to eliminate. The apparatus disclosed by Thong allows the user to define a plurality of sequential events and logically connect them to each other through the use of a state machine, logic, and variable delay devices. Because the events are interrelated over time by these delay lines, events which occur during the delay intervals do not influence the resulting trigger generation. A consequence of this is that when the selected sequential events are temporally non-contiguous, the intervals between events are automatically in effect "don't care" from the user's point of view, whether this is what the user wishes or not.
What is desired is an approach to detection of sequences of events in serial digital data in which the intervals between signal transitions are explicitly dealt with, thus allowing the precise and continuous monitoring of the signal in question so that all of its behavior may be more accurately specified by the user as he defines the pattern to be recognized.